Electrical musical instrument



DCC 9, 1947- J. M. HANERT Er Al.

ELECTRICAL MUSICAL INSTRUMENT Filed June 20, 1942 2 sheets-Sheet 1 www Nh N KXN .af/m r wm w v nnn mMd/W w ma. /Q Z @J k om Dec. 9, 1947. J, M. HANERT Er AL 2,432,152

ELECTRICAL MUSICAL INSTRUMENT Filed June zo", 1942 2 sheets-sheet z and alffczHa/zcoc/c Patented Dec. 9, 1947 ELECTRICAL MUSICAL INSTRUMENT John M. Hanert, Wilmette, and David Hancock,

Jr., Chicago, Ill., assignors to Hammond Instrument Company, Chicago, Ill., a corporation of Delaware Application June 20, 1942, Serial No. 447,770'

(Cl. Sei-1.19)

13 Claims. l

Our invention relates generally to electrical musical instruments, and more particularly to improvements in electrical sound signal generating and controlling means for instruments of this type.

Many musically useful sounds co-nsist of vibrations in which the pitch is not so clearly defined as to suggest a particular note in the even tempered scale. Musical effects such as produced by snare drums, kettle drums, tympani and o-ther percussion instruments and traps are of this character. In the instrument of our invention such effects may be produced and, in addition, it is possible to produce sounds of more denite pitch, for example, sounds simulating a chorus of sibilant whistles.

It is thus an object of our invention to provide an improved signal generator for electrical mu sica] instruments which is capable of producing interesting, novel, and musically useful rhythmic sound effects.

A further object is to provide improved control circuits and means for tonal modication of the output of a complex signal generator containing all audio frequencies.

A further object is to provide a simple and easily operable electrical musical instrument which may be used in orchestras to take the place of drums and traps, and the manufacturing cost oi which is a small fraction of the cost of a mechanical musical instrument capable of producing equivalent musical results.

A further object is to provide an improved electronic signal generator capable of producing a wide spectrum of audio frequency signals.

A further object is to provide an improved super-regenerative electronic signal generator operating on the random noise principle for the production of a Wide spectrum of audio frequencies.

Other objects will appear from the following description, reference being had to the accompanying drawings in which:

Figure l is a Wiring diagram of a preferred embodiinent of the invention; and

Figure 2 is a perspective View of a complete instrument embodying a part of the invention.

The instrument of our invention may be described generally as comprising a random noise generator forming a source of a wide range of frequencies, of which those in the .audio range are utilized. The generator of these frequencies may assume a Wide variety of forms, but as disclosed herein, comprises a novel type of superregenerative oscillator operating at superaudio frequencies and in which the audio frequencies are produced by thermal agitation, vacuum tube noises, or other factors which result in random discontinuities or irregularities in current flow.

The output 0f the generator is selectively filtered by tuned iilter meshes, ampliiied, selectively controlled as to amplitude or intensity en- Velope, controlled as to over-all intensity level, and translated into sound.

In one form of the invention there may be two sets of playing keys, one set for selecting any one of a plurality of liltering meshes which pass narrow bands of frequencies to the output of the instrument, and the other set selectively rendering broadly tuned ltering meshes effective and controlling the transmission of the signal to the speaker. The latter set of keys may be arranged in groups of two or more, each group being associated with a single filtering mesh, and the keys of a group arranged to cause the production of sounds of the same quality but of different degrees of intensity.

An optionally usable means is provided for introducing a transient electrical impulse of musically desirable tone quality into the output of the instrument as an incident to the start of the tone.

The instrument is adapted for use in orchestras, and in general for the rhythmic accompaniment of other instrumental and vocal music. Generally, the instrument comprises a generator I0 which operates to generate all of the frequencies present in the audio spectrum, chromatic controls l2 comprising resonant filter meshes, decoupling and preamplifier stage I4 (comprising triodes i2 and M), rhythm percussion controls rand filtering meshes I6, envelope control circuits i3, a power ampliner stage 2Q, and a speaker 22.

The generator I@ comprises a pentode 24, such as a 7B5 type vacuum tube having a grounded cathode 26 and grounded suppressor grid 2l, a control grid 28, a screen grid 29, and a plate Bil. The tube 24 operates in a manner similar to a super-regenerative amplier. The grid 28 is connected to ground through a biasing and relaxation timing resistor RI of suitable value.

The plate circuit of the tube 24 comprises an inductance Ll and a condenser CII in parallel, the inductance Li having a suitable tap 32 connected to a source of plate potential such as +300 V. through a load resistor R2. The screen grid 29 may also be effectively connected to the tap 32. A superaudio frequency by-pass condenser CI2 is connected between the tap 32 and ground. The resonant mesh Ll, Ci l, tuned to a radio frequency, is connected between the plate 30 and a regenerative feedback circuit including a conductor 34 and a condenser C13, the latter also serving as a timing condenser for controlling the rate of relaxation or quenching of the radio frequency oscillations. An audio frequency signal is derived from the generator I by way of a con ductor 36 connected to the tap 32, and is transmitted to a terminal 38 through a blocking condenser CI4 and voltage divider resistors R3, R4.

The resonant frequency of the mesh LI, CI I is preferably of radio frequency while the frequency of relaxation is determined by the values of RI and CIS, these Values being selected so that relaxation or quenching takes place at a lower, but nevertheless superaudible, frequency. The generator l0 thus tends to oscillate at a radio frequency, but such oscillation is periodically interrupted by the quenching potential impressed upon the grid 28 due to the time constant of a grid bias determining mesh consisting of RI and CIS. The latter mesh is representative of any suitable means for impressing a quenching potential upon the grid 28. If the condenser CI3 were made small enough relative to the resistance RI so as to prevent storage of grid bias for a time less than the time of one wave of the radio frequency oscillations (whose natural frequency is that of the resonant mesh consisting of LI and CI I), the generator Ill would constitute an ordinary radio frequency oscillator producing a signal on terminal 32 consisting substantially only of the radio frequency.

It is the purpose of the generator I0 to take advantage of the audio frequency spectrum generated as a result of random thermal agitation occurring primarily in the inductance LI, but also produced in the thermionie tube, generally termed tube noise. As a result of such random thermal agitation, all audio frequencies are present, since the Voltage gain of the system, between the grid 28 and the plate 30 of the tube 24, approaches innity. As is well known in the radio art, the gain of the tube 24 may be enormously increased by applying regenerative feedback between the plate circuit and the grid circuit through application of a signal of the proper phase from the inductance LI to the grid 28. As the amount of this feedback is increased, the gain will rise until a point is reached where increase in positiveness of the grid 28 does not cause a corresponding increase in plate current. Under these conditions, self oscillation will commence, and the system as a whole can no longer be considered as a regenerative amplifier, but becomes an oscillator.

Just prior to the point at which the grid approaches the positiveness necessary to cause self oscillation, the voltage gain of the system may be considered as being infinite-that is, an infinitely small amount of voltage applied to the grid causes a gain in the plate current. Under these conditions of gain, in which the amplification factor approaches infinity, the current through the inductance LI wn] become exceedingly high and all inherent noises such as thermal agitation in the plate inductance LI will be greatly amplified. Due to the usual inherent instability of these conditions, it is desirable to provide stabilizing means. Stabilization is achieved by selecting the condenser CIS and resistor RI of such values that they have a time constant considerably in excess of the period of the radio frequency wave determined by the mesh comprising the inductance LI and condenser CII. This rate of relaxation being superaudible, it Will serve solely as a stabilizing means for increasing the amount of regenerative feedback applied to the grid 23, without causing the tube to become selfoscillatory at the radio frequency. Thus, the gain may be exceedingly high, and a noise spectrum containing all audio frequencies will be prseent at the tap 32, The condenser CI2 is of such size as effectively to shunt the radio and high fundamental relaxation frequency and harmonies thereof, leaving only the noise spectrum containing all audio frequencies to be transmitted along the conductor 36.

The junction 38 of the voltage divider resistances R3 and R4 is adapted to be connected by a switch 40 directly with the input of a triode 42 of one of the decoupling and preamplifier stages I4. When the switch 40 is moved to its dotted line position, the triode 44 has its input effectively connected to the terminal 38. The triodes 42 and 44 may be contained in a common envelope constituting a twin triode tube which may be of the 7N7G type.

The triode 44 comprises a cathode 46 connected to ground through a self bias resistor R13, a grid 50 connected to the junction 38, and a plate 52 connected to a suitable source of operating plate potential indicated as +300 v. through a load resistor R5. A bus bar 54 is connected to the plate 52 through a blocking condenser CI5. A plurality of playing keys C2 to C3, bearing reference characters corresponding to the musical notation of the tones controlled thereby, are adapted to operate switch arms 56, The keys C2 to Cfl are intended to be illustrative of any suitable ra^ge of keys such for example as from C2 to C5 inclusive, which corresponds to a frequency range of from 130.81 to 1046.50 C. P. S.

Each of the switches 5G is connected by a decoupling resistor RG to a sharply tuned resonant mesh comprising an inductance L2 and condenser Cl. These tuned meshes are of course of different resonant frequencies corresponding respectively to the frequencies of the notes of the musical scale. One side of each of these meshes is connected to ground, while the other side thereof is connected through a second decoupling resistor Rl to a conductor 58. Decoupling resistors R5 and Rl preferably are made large by comparison with the impedance of the tuned meshes L2, Cl at their respective resonant frequencies. Inductances L2 and condensers CI are of such values that the impedances of the various meshes are approximately equal at their respective resonant frequencies.

The conductor 58 is connected by the switch 40, when the latter is in its dotted line position, with the grid 69 of triode 42, the grid being also connected to ground through a grid resistor R8, the resistor R8 preventing the grid 6U from oating during the interval that the switch 40 is being shifted. The cathode 62 of the triode 42 is connected to ground through a biasing resistor R8, while the plate 64 of this tube is connected to a suitable plate potential source indicated as +300 v, through. a load resistor RII). The output of the tube 42 is fed to the primary winding 66 of a transformer TI through a decoupling resistor REI and a blocking condenser C8, the primary winding B6 being shunted by a load resistor RI l.

The frequency spectrum of the signal impressed across the primary 6B of the transformer TI is suitably modified by the key operated rhythm percussion controls I6, the signal being supplied to these controls through a conductor 68. A plurality of rhythm control keys KI to KI2 are representative of any suitable number of keys which may be desired in any particular instrument. Each of these keys is adapted successively to operate a pair of switches, the lirst operated switch comprising switch arms 'Ill and 'II and the second operated switch comprising switch arms 'l2 and 73. The arrangement is such that when a switch arm 'it is depressed by its key to make contact with the switch arm 'I the latter, through an insulating button lll, presses its switch arm I2 into contact with the switch arm '13. The switch arms l of all of the key operated switches are connected to the conductor 68, while switch arms 'il are connected in pairs, each to a terminal of one of a plurality of frequency responsive meshes, the other terminals of which are connected to a grounded conductor 76. f

The frequency responsive mesh for the keys KI and K2 comprises a resistor RIZ, while that for the pairs of keys Kit-K4, Ki-K, ICI- K8 comprise inductances L2, each in parallel with a condenser C9, these meshes being broadly tuned to widely separated audio frequency bands. The switch arms ill for the keys K, Kl are connected to a ltering mesh consisting solely of an inductance L3, while the ltering mesh for the keys KIi and KEZ consists solely of a condenser Inductance L3 causes the signal to contain predominantly high frequencies, while condenser CI@ causes the signal to contain predominantly lower frequencies.

All of the switch arms 'I3 are connected to a common conductor 88 which, as will appear hereinafter, controls the amplitude of the output of the instrument The switch arms I2 for the keys KI, K3, K5, etc., are connected to a common conductor 82 which is connected to a terminal indicated as +125 v. of a voltage divider, the latter comprising series resistances RI4 and RI5 connected between a suitable potential source indicated as +300 v. and ground. The switch arms I2 for the keys K2, K4, K6, etc.,

are connected to a common conductor 84 which is connected to a slide contact 86 operating on the potentiometer resistance RI 5.

The secondary winding 88 of the transformer TI has a center tap connected to a suitable potential indicated as l v. and its end terminals connected to the grids di) and 3i of variable mu pentodes 92, Q3 in a push-pull manner. The tubes 92, s3 are preferably of the 'lA'l type. rlhe cathodes Se, Q of these tubes and their suppressor grids 35, 91 are connected to ground. The screen grids 98, 99 of these tubes 92, 93 are connected to the common conductor 8f3 and are connected to ground through an attack and de cay control condenser Ci8. The plates IDU, HEI or the tubes 92, 93 are connected to a plate potential source indicated as +300 v. through load resistors Rl and RI? respectively, and are connected through blocking condensers C and C2! respectively with the input circuits of power amplifier tubes IEIZ, I03. f

The grids Ide, E95 of the tubes Ii2, E93 are connected to ground through grid resistors RI, Ri respectively, and the cathodes I, it of these tubes are connected to ground through a self-biasing network comprising resistor R2!) and condenser C22. The plates I IQ, I i I of these tubes are connected respectively to the ends of the primary winding of the output transformer T2, the center tap of which is connected to a suit- 6 able source of plate potential indicated as +300 v. The screen grids II2, IIII of these tubes are maintained at operating potential through a protective resistor R22 which is connected between these screen grids and the +300 v. terminal.

The secondary of the transformer T2 is connected to the speaker 22 through an adjustable resistor R24 lwhich may be utilized as the output volume control of the instrument as a whole.

Whenever any one of the keys KI to KI2 is depressed to close its switch 12, "i3, a potential from the voltage divider R14, RI5 is impressed upon the screen grids 98 and 99, and the rate at which the potential on these screen grids builds up is determined by the values of the resistors RIA, RIE witlcl respect to the capacity of the condenser CIS and the screen grid circuit impedances of the tubes 92, 93. rI'hus, for example, when the keys KI, K3, K5, etc., are operated, a potential of volts will be impressed upon the screen grids 98, 99, this potential corresponde ing to the maximum operating potential for these remote cutoff tubes, which may be of the 7A? type. Uponcperation of any of the keys K2, K4, K5, etc., a potential of lesser value, determined by the position of the slide contact 85, will be impressed upon the screen grids 98, 99, so that the gain of these tubes will be substantially less than its maximum value. Thus, depression of any of the latter group of keys will result in the production of a sound of less intensity than when any of the keys of the group KI, K3, K5, etc., is depressed.

Under certain circumstances it may be desirable to introduce a transient incidental to the starting of a sound upon depression of one of the keys KI to KIZ so as to produce the effect of a pronounced percussive attack. This may be accomplished by utilizing the relatively abrupt change in the screen voltage to affect the output of one only of the tubes 92, 93. Such means is herein shown as comprising a switch |253 connected to the conductor 8B which is optionally operable to connect any one of a plurality of meshes between the conductor 86 and the plate IUI of the tube 93, these meshes being illustrated as each comprising a condenser C26 connected in series with a resistor R26. These meshes have diierent impedances and diierent frequencies at which the impedances of the elements of the meshes match. By making this point of impedance match at a high frequency and also by making the mesh impedance high, a relatively soft high pitched transient will be produced, whereas if the reactance of the condenser C26 matches the resistance of the resistor R26 at a low frequency and the mesh impedance is relatively low, a loud low pitched transient will be produced.

In Figure 2 we have shown the case of an instrument of the type previously described but in which the chromatic controls I2 and associated parts are omitted, this instrument being primarily for the production of rhythmic percussive embellishments. This instrument comprises a case |30 having a cover I 32, inside of which a music rack |34 may be provided. The speaker 22 of the instrument is located within the casing |30, a grille ISS being provided in front and a similar grille at the rear of the casing to permit egress of the sound. The instrument is suiciently light thatiit is easily portable and is thus adapted to be connected `to a suitable current outlet by an extension cord and plugl |38. The

top of the body of the casing |30 is provided by a keyboard plate |40 upon which are mounted the controls.

The controls consist of an over-all volume control lever |42 which operate the Variable resistance R24, a percussion control lever |44 which operates the switch |20, an accent control lever |46 which operates the slide contact 86, and an On-OIT switch lever |48 which controls the application of power to the power supply system of the instrument. The instrument is provided with small keys KI to KI 2, the odd numbered keys being marked f, while the even numbered keys are marked p, representing the forte and piano. The keyboard arrangement of the instrument is very compact so that with a little practice the player may, if he desires, play the instrument with one hand and thus has the other hand free for playing triangles, tambourines and similar instruments which are normally played with one hand by the drummer.

In playing the instrument, it will be assumed that the switch 40 is in its dotted line position. As previously described in some detail, the output of the generator I consists of audio frequencies which are impressed upon the grid 50 of the decoupling and amplifying tube 44. The gain of the tube 44 is suicient to compensate for the losses incurred chiefly in the decoupling resistors R6 and RI as well as in the resonant meshes associated therewith. Assuming, for example, that the key C2 is depressed, closure of its switch 56 will result in providing a low impedance path to ground for all frequencies except the frequency of 130.81 cps and to a lesser extent, other frequencies very close to this frequency. Thus, the signal appearing in the conductor 58, and impressed upon the grid 60 of the preamplifier tube 42, will be of a denite recognizable pitch. In order that the envelope control .tubes 92, 93 may conduct the signal it will be necessary for the player to depress one of the keys KI to KIZ so as to put an operating potential upon the screen grids 98, 99.

The keys KI to KIZ may be provided with a suitable optionally operable flexible keyboard mechanism (similar to that used on adding machines), whereby each key is held depressed until released by the depression of another key. A key latching and releasing mechanism such as that if shown in the patent to Laurens Hammond No. 1,956,350 associated with the preset keys thereof .may be utilized, preferably with some additional means for rendering the latch means inoperative whenever desired.

Assuming that the key KI has thus been depressed and is held depressed by the latching mechanism, the full operating screen potential will be applied to the control tubes 92 and 93, and the signal will be transmitted through the power amplifier and translated into sound by the speaker 22. The fact that depression of the key KI results in connecting the resistor RIZ in parallel with the resistor RII will not have any pronounced eiect upon the character of the sound produced. If it is desired to change the intensity of the sound output without adjustment of the volume control resistor R24, the player may depress one of the keys K2, K4, etc., whereupon a lower potential of value determined by the adjustment of the slide contact 86 will be impressed upon the screen grids 90 and 99. Thus, by simultaneous operation of selected keys of the KI to KIZ group with the keys of the C2 to C3 group, the player may accent certain chords or melody notes as played. Interesting quality and dynamic accents may be thereby imparted to the music.

When the instrument is operated with the switch 40 in full line position, the chromatic controls `I 2 are rendered inoperative, and under these circumstances it will normally be desirable to render the latching mechanism for the keys KI to KIZ inoperative. Under these conditions the output of the generator I0 is ampliiied by the triode 42 and contains the full range of all audio frequencies. (The instrument shown in Fig. 2 has all oi the circuits and parts shown in Fig. 1 except the chromatic controls I2 and the tube 44, and has a fixed connection corresponding to the switch 40 in full line position.)

Thus, the full range of audio frequencies is present between ground and the conductor 68 so that upon depression of any one of the keys KI to KIZ, a range of frequencies, determined by one of the meshes RIZ, L2-C9, L3, or CIO, is connected across the terminals of the primary winding 66 of the transformer TI Depression of one of the keys KI to KIZ also results in the impression of an operating potential upon screen grids 98, 99 of the envelope control tubes 92, 93. The series of keys KI, K3, K5, etc., will cause the full operating potential of +125 v. to be impressed upon these screen grids, whereas by control oi the position of the slide contact 86 a lesser potential of selected value may be impressed upon the screen grids 98, 99 upon the depression of any of the keys of the series KZ, K4, K6, etc.

When the switch |20 is not connected to any one of the series meshes C26, R26 the attack of the sound produced will not be too abrupt because of the provision of the condenser CIB which delays the application of the full operating potential to the screen grids 98, 99. Similarly, upon release of a depressed key the condenser CI8 will form a reservoir for screen current so that the sound will have a substantially transientless decay. When the player desires a very pronounced percussion eiiect the switch |20 will be shifted to connect the conductor with one of the meshes C26--R26. Thus, the change in D, C. potential on the conductor 80 resulting from the depiession of a key will produce a transient in the output of the tube 93, and this transient will result in a decided percussive click at the start of the sound, the degree of this eiect depending upon which of the C2G-R26 meshes is employed.

The player of the instrument will depress the keys in a staccato manner to provide the rhythm accompaniment for an orchestra, playing the trap drummers musical score, and the sounds produced by the instrument may be made to simulate those obtained from bass drums, kettle drums, and various drummers traps. The player has available to him Very convenient means for accenting the beat by playing the unaccented note on the key KZ, for example, and depressing the key KI to produce the accented note. In addition, the player may slowly or rapidly operate the volume control R24 to accord with the expression required in the selection being rendered. Thus, in playing the instrument one hand may be used to depress the playing keys, while the other is utilized to operate the various controls, and thus the player is enabled to produce a wide variety of rhythmic effects.

The action of the keys KI to KIZ, as well as the keys C2 to C3, is made as light and fast as compatible with substantial durable construction, so that the player may produce unusual and complicated rhythm patterns in depressing the keys. The player may thus produce more rapid and intricate rhythm patterns than by the use of mechanical percussive instruments, since keys may be depressed more rapidly by the fingers than the trap drummer can play upon a number of spatially separated drums and traps.

When any one of the keys Kl to KIZ is held depressed, the effect of a group of drums being rolled is obtained.

Furthermore, the operation of the instrument is substantially effortless from the physical point of View so that the player may use the instrument for extended periods without becoming physically tired. Since the intensity of the sounds produced is controlled merely by shifting the volume control when p-laying upon different keys, it is readily apparent that loud sounds may be produced with just as little effort as are sounds of low intensity. Loud musical passages may be played with a facilitythat is usually obtainable by the use of mechanical percussive instruments only when such passages are played at low intensity.

Except possibly for the random noise generator, the values of the circuit elements and the mechanical construction of the parts will be readily apparent to those skilled in the art. The random noise generator operates upon radically novel principles and it may therefore be helpful to Aset forth, as an example, the values of the principal component parts of a generator which have been found to be practical. The tube 25, was of the 7B5 type; resistor RI, 250,000 ohms; resistor R2, 50,000 ohms; condenser C13, .000i microfarad; condenser CIZ, .006 microfarad; condenser CII, micro-microfarads; and the inductance LI was 20 turns of .5 diameter coil having an air core.

We have shown and described the instrument as having an electronic noise generator, but it will be clear to those skilled in the art that any other suitable source of noise may be employed, such for example, as a phonograph record and pickup in which the record has a random noise recorded thereon, a continuously operating noise produced by any suitable pneumatic or mechanical device in which the frequencies of the audio spectrum are present, or the noise produced by gaseous ionization.

Thus the term random noise or random noise frequencies is intended to refer to a sound, or an electrical signal representing sound, in which a large number and wide range of generally unrelated frequencies are present, which is perceived by the listener as noise, and which does not have any definite musical pitch. A random noise is therefore non-cyclical or nonrepetitive.

While we have shown and described a particular embodiment of the invention it will be apparent to those skilled in the art that the invention is susceptible of embodiment in a wide variety of forms. We therefore Wish, in the following claims, to include Within the scope of our invention all such modifications and variations by which substantially the results of our invention may be obtained by the use of substantially the same or equivalent means.

We claim:

l. In an electrical musical instrument having a signal transmission system and an eiectroacoustic translating means, the combination of an electrical random noise signal generator, means associated with the signal transmission system for predetermining the amplitude of the signal output thereof, a plurality of frequency selective filtering meshes, playing keys, and means operated by said playing keys to couple said generator to the input of the transmission system utilizing one or more of said filtering meshes to determine the frequencies transmitted from said generator to the signal transmission system.

2. In an electrical musical instrument having electroacoustic translating means, an electrical random noise signal generator, a signal transmission system coupling said generator to the translating means, a plurality of frequency responsive meshes, and key operated means for selectively coupling said meshes in said transmission system, thereby to change the frequency transmission characteristics of said system.

3. In anelectrical musical instrument having a signal generating system, an amplifying and electroacoustic translating system, a transmission system for coupling the generating system to the amplifying and electroacoustic translating system, said transmission system including an electron discharge device having an electrode the potential of which determines the gain thereof, a plurality of playing keys for impressing an operating potential upon said electrode of said device, and means operated by said keys and including a frequency selective mesh for impressing a direct current transient upon the output of said device, whereby a musical transientY tone may be produced as an incident to the start of a tone produced from the signal supplied by said generating system.

4. In an electrical musical instrument having an electroacoustic translating system, an electrical signal generating system, and an electron discharge device forming a signal intensity determining part of a coupling between said generating system and said electroacoustic translating system, a plurality of playing keys, and switches operated by said keys to impress a direct current transient potential upon the output of said electron discharge device upon operation of said keys, thereby to add to the signal derived from said generating system an electrical signal corresponding to a percussive tone.

5. In an electrical musical instrument having an output system, the combination of an electrical generator of signals, a direct current operated envelope determining means effective to cause transmission of the signal from said generator to the output system, playing keys, switches operated by said keys to render effective the transmission of said signal from said generator to the output system by applying an operating direct current potential to said direct current envelope determining means, a common connection between said key operated switches, and frequency selective means coupled to said common connection and to said output system to produce a musically desirable transient in the output system.

6. In an electrical musical instrument having a plurality of playing keys corresponding to the intervals of the musical scale and having an electroacoustic translating means, the combination of a generator having an electrical signal output containing a wide range of audio frequencies differing from each other by intervals of a fraction of a semi-tone, a plurality of sharply resonant filtering meshes, and means controlled by said keys for selectively utilizing one or more of said meshes as a part of a coupling betWeerl Said gencrater and said electroacoustic translating means.

7. In an electrical musical instrument for producing percussive effects and having an amplier and electroacoustic translating means, the combination of a generator of a full range of audio frequencies differing from each other by small fractions of a semi-tone, a plurality of frequency selective meshes, and selectively operable means for utilizing said meshes to couple said generator to said electroacoustic translating means.

8. In an electrical musical instrument a generator of electrical random noise frequencies, electroacoustic translating means, a plurality `of frequency discriminatory meshes, and means to selectively transmit to said electroacoutic translating means the signal produced by said generator as modified by one of said meshes;

9. In an electrical musical instrument having playing keys and an electroacoustic translating system, a random noise signal generator, a plurality of resonant meshes, means operated by said keys selectively to render said meshes effective to modify the output of said generator, signal transmission means for coupling said generator tc said electroacoustic translating system, and means operated by said keys for rendering said transmission means effective.

10. In an electrical musical instrument having playing keys and electroacoustic translating means, a random noise signal generator, a plurality of resonant meshes, means operated by said I keys selectivelyV to render said meshes effective to modify the output of said generator, and signal transmission means for coupling said generator to said electroacoustic translating means.

11. A random noise generator for the production of a continuous audio frequency spectrum comprising an electron discharge device having an input circuit and an output circuit, a regenerative circuit between said input and output circuits, a reactive mesh in the output circuit of said device, said mesh being resonant at a high superaudio frequency and causing said device and associated circuits to oscillate at a high superaudio frequency, means coupled with the input circuit of said device to cause periodic interruption of such high superaudio frequency oscillation at a lower superaudio frequency, and means including a superaudio frequency by-pass mesh for coupling said device to a signal utilization circuit.

12. A random noise generator for the production of a continuous audio frequency spectrum comprising an electron discharge device having an input circuit and an output circuit, a regenerative circuit between said input and output circuits, a reactive mesh in the output circuit of said device to cause said device and associated circuits to oscillate at its high superaudio frequency, means coupled with the input circuit of said device to cause periodic interruption of such high superaudio frequency oscillation at a lower superaudio frequency, and an electrical filter coupled to the output circuit to remove from the output signal substantially all of the superaudio frequencies and to transmit substantially all of the audio frequencies.

13. In an electrical musical instrument having a signal generating system, an amplifying and electroacoustic translating system, a transmission system for coupling the generating system to the amplifying electroacoustic translating system, a plurality of playing keys for controlling the transmission of electrical signals from the generating system to the translating system, and means operated by said keys for impressing a transient electrical impulse upon the transmission system as an incident to the start of the transmission of a signal from the generating system to the translating system.

JOHN M. HANERT. DAVID HANCOCK, JR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,095,707' Kucher Oct. 12, 1937 2,161,706 Hammond June 6, 1939 2,212,292 Kock Aug. 20, 1940 2,254,284 Hanert Sept, 2, 1941 

